feat: add day 3 - part 2

day_3/src/lib.rs

@@ -1,8 +1,17 @@
-#[derive(Debug, PartialEq)]
+use std::collections::HashMap;
+
+#[derive(Debug, PartialEq, Default, Clone, Copy)]
+pub struct GearPosition {
+    pub index_line: usize,
+    pub index_character: usize,
+}
+
+#[derive(Debug, PartialEq, Default, Clone, Copy)]
 pub struct NumberPosition {
     pub index_start: usize,
     pub index_end: usize,
     pub value: usize,
+    pub gear_position: GearPosition,
 }
 
 pub fn get_numbers_positions_from_line(line: &str) -> Vec<NumberPosition> {
@@ -23,6 +32,7 @@ pub fn get_numbers_positions_from_line(line: &str) -> Vec<NumberPosition> {
                 index_start,
                 index_end,
                 value,
+                gear_position: GearPosition::default(),
             });
             number_string = String::from("");
             index_start = 0;
@@ -35,6 +45,7 @@ pub fn get_numbers_positions_from_line(line: &str) -> Vec<NumberPosition> {
             index_start,
             index_end,
             value,
+            gear_position: GearPosition::default(),
         });
     }
     result
@@ -44,6 +55,10 @@ pub fn is_symbol(character: char) -> bool {
     !character.is_ascii_digit() && character != '.'
 }
 
+pub fn is_gear_symbol(character: char) -> bool {
+    character == '*'
+}
+
 pub fn part_1(input: &str) -> usize {
     let lines = input.lines().collect::<Vec<&str>>();
     lines
@@ -82,15 +97,108 @@ pub fn part_1(input: &str) -> usize {
                         || has_symbol_on_the_top
                         || has_symbol_on_the_bottom
                 })
-                .map(|matching_number| matching_number.value)
+                .map(|number_position| number_position.value)
                 .sum::<usize>()
         })
         .sum()
 }
 
-// pub fn part_2(_input: &str) -> usize {
-//     42
-// }
+pub fn part_2(input: &str) -> usize {
+    let lines = input.lines().collect::<Vec<&str>>();
+    let mut number_positions = vec![];
+
+    lines.iter().enumerate().for_each(|(index_line, &line)| {
+        get_numbers_positions_from_line(line)
+            .iter()
+            .for_each(|&number_position| {
+                let index_start = number_position.index_start.saturating_sub(1);
+                let index_end = if number_position.index_end + 1 >= line.len() {
+                    line.len().saturating_sub(1)
+                } else {
+                    number_position.index_end + 1
+                };
+
+                let has_symbol_on_the_left =
+                    line.chars().nth(index_start).is_some_and(is_gear_symbol);
+                if has_symbol_on_the_left {
+                    let mut number_position = number_position.to_owned();
+                    number_position.gear_position.index_line = index_line;
+                    number_position.gear_position.index_character = index_start;
+                    number_positions.push(number_position);
+                }
+
+                let has_symbol_on_the_right =
+                    line.chars().nth(index_end).is_some_and(is_gear_symbol);
+                if has_symbol_on_the_right {
+                    let mut number_position = number_position.to_owned();
+                    number_position.gear_position.index_line = index_line;
+                    number_position.gear_position.index_character = index_end;
+                    number_positions.push(number_position);
+                }
+
+                let index_character_top =
+                    lines.get(index_line.saturating_sub(1)).and_then(|&line| {
+                        line.get(index_start..=index_end)
+                            .and_then(|value| value.chars().position(is_gear_symbol))
+                    });
+                if let Some(index_character_top) = index_character_top {
+                    let mut number_position = number_position.to_owned();
+                    number_position.gear_position.index_line = index_line.saturating_sub(1);
+                    number_position.gear_position.index_character =
+                        index_character_top + index_start;
+                    number_positions.push(number_position);
+                }
+
+                let index_character_bottom = lines.get(index_line + 1).and_then(|&line| {
+                    line.get(index_start..=index_end)
+                        .and_then(|value| value.chars().position(is_gear_symbol))
+                });
+                if let Some(index_character_bottom) = index_character_bottom {
+                    let mut number_position = number_position.to_owned();
+                    number_position.gear_position.index_line = index_line + 1;
+                    number_position.gear_position.index_character =
+                        index_character_bottom + index_start;
+                    number_positions.push(number_position);
+                }
+            });
+    });
+
+    // Key: "index_line-index_character"
+    // Value: usize
+    let mut gear_positions: HashMap<String, Vec<usize>> = HashMap::new();
+
+    number_positions.iter().for_each(|&number_position| {
+        let key = format!(
+            "{}-{}",
+            number_position.gear_position.index_line, number_position.gear_position.index_character,
+        );
+        match gear_positions.get_mut(&key) {
+            Some(gear_positions) => {
+                gear_positions.push(number_position.value);
+            }
+            None => {
+                gear_positions.insert(key, vec![number_position.value]);
+            }
+        }
+    });
+
+    let mut result = 0;
+
+    for (_, value) in gear_positions.iter() {
+        if value.len() != 2 {
+            continue;
+        }
+
+        let first_number = value.first();
+        let second_number = value.last();
+
+        if let (Some(first_number), Some(second_number)) = (first_number, second_number) {
+            result += first_number * second_number;
+        }
+    }
+
+    result
+}
 
 #[cfg(test)]
 mod day_3_tests {
@@ -101,8 +209,8 @@ mod day_3_tests {
         assert_eq!(part_1(include_str!("../input_example_1.txt")), 4361);
     }
 
-    // #[test]
-    // fn test_part_2_example() {
-    //     assert_eq!(part_2(include_str!("../input_example_1.txt")), 467835);
-    // }
+    #[test]
+    fn test_part_2_example() {
+        assert_eq!(part_2(include_str!("../input_example_1.txt")), 467835);
+    }
 }

day_3/src/main.rs

@@ -1,8 +1,8 @@
-use day_3::part_1;
+use day_3::{part_1, part_2};
 
 fn main() {
     let input = include_str!("../input.txt");
     println!("- Day 3: Gear Ratios -");
     println!("Answer Part 1: {}", part_1(input));
-    // println!("Answer Part 2: {}", part_2(input));
+    println!("Answer Part 2: {}", part_2(input));
 }